1. Field of the Invention
The present invention relates to mechanical structures for supporting piezoelectric resonators, and in particular to a dismountable device for supporting a piezoelectric resonator inside a casing, said device comprising four independent and identical U-shaped coupling flanges made of a material whose expansion coefficient in an axial direction with respect to said resonator is substantially identical to that of the material constituting said resonator, each said coupling flange cooperating with a pair of springs, each spring being formed of circular cross-section metal wire made of a high yield strength alloy, the thus constituted set of coupling flanges and springs being symmetrical with respect to two mutually perpendicular axial planes of said resonator and with respect to a center plane of said resonator.
2. Prior Art
Various examples of piezoelectric resonators are already known.
According to one conventional example, a crystal resonator is constituted by a planoconvex or biconvex quartz plate, of circular contour, on the front faces of which plate are directly deposited metallic electrodes, known as adhering electrodes.
According to another example, the quartz plate with adhering electrodes is limited on the outside by a quartz ring used for supporting the crystal resonator.
According to yet another example U.S. Pat. No. 4,705,982, the crystal resonator is connected to a quartz ring via bridges cut through the mass. The electrodes may be adhering or non-adhering. In the latter case, the electrodes are deposited over quartz plates having the same contour and shape as the crystal, and only the rings of the plates supporting the electrodes are in contact with the ring of the vibrating part of the crystal.
Irrespective of the type of resonator used, it is generally necessary to incorporate the resonator within a protective casing which, if sealed, further allows it to enclose a high or low vacuum which improves the resonator's operating conditions, and hence its performance characteristics.
The casing and crystal resonator do not, however, have the same mechanical and thermal characteristics, making it therefore necessary to provide a special mechanical structure in view of limiting the effect of external perturbations and introducing a minimum amount of stress at the center of the crystal.
It has already been proposed in document U.S. Pat. No. 4,705,982 to provide a device for supporting a piezoelectric resonator inside a casing that is suitable for different types of piezoelectric resonators and designed to minimize the resonator's relative frequency variations due to accelerometric, thermal or pressure induced perturbations, so as to give maximum stability and lowest possible aging effects to the resonator.
According to the above supporting device, there are provided first and second identical half-shells for positioning the resonator, the said half-shells being in a material having the same nature and design as the material constituting the resonator, and each one being cross-shaped and comprising four branches of which the free ends define angle pieces with a horizontal bearing surface of small dimensions, which surface is slightly raised with respect to the central part of the corresponding half-shell as well as one vertical branch holding the edge of the resonator and four identical independent securing flanges to secure the two half shells in position, said flanges having the shape of a down-turned U, each flange comprising, at the end of each branch of the U, two stop elements bearing against two superposed branches of the two half-shells of the bearing faces of those branches facing said horizontal bearing surfaces and cooperating with at least four pairs of springs for suspending the four securing flanges with respect to the casing.
In such a known resonator supporting device, the springs for suspending the coupling flanges holding the cross shaped half-shells are joined directly to the casing and fixed by one of their ends to the junction between the different walls forming the casing. This prevents a complete uncoupling between the housing--whose main purpose is to maintain the vacuum within its enclosing space, and which can be submitted to shocks or distortions--and the suspension system itself, which must remain as symmetrical as possible. Besides, the manufacture of cross shaped half-shells in a material having the same nature and design as that of the resonator introduces a cost penalty.